Carbon nanotubes are tubular carbon polyhedra having a structure in which a graphite sheet is formed in a closed cylindrical shape. Examples of these carbon nanotubes include multi-walled nanotubes having a multi-walled structure in which a graphite sheet is formed in a closed cylindrical shape, and single-walled nanotubes having a single-walled structure in which a graphite sheet is formed in a closed cylindrical shape.
Multi-walled nanotubes were discovered in 1991 by Iijima. More specifically, multi-walled nanotubes were discovered to exist in a mass of carbon deposited on the cathode in an arc discharge method (Non-Patent Document 1). Subsequently, multi-walled nanotube research was actively pursued, so that it is now possible to synthesize multi-walled nanotubes in large quantities.
On the other hand, single-walled nanotubes have an inner diameter of roughly about 0.4 to 10 nanometers (nm). Synthesis of single-walled nanotubes was simultaneously reported by Iijima and an IBM group in 1993. The electron state of single-walled nanotubes has been theoretically predicted, and the electron physical properties are thought to change from a metallic nature to a semiconductor-like nature due to a helical rolled arrangement. Therefore, single-walled nanotubes hold promise as a future electronic material.
Examples of other single-walled nanotubes applications include as a conductive composite material, a nano-electronic material, a field electron emission emitter, a high directivity radiation source, a soft X-ray source, a one-dimensional conducting material, a highly heat-conductive material, a hydrogen storage material and the like. Moreover, it is believed that the addition of functional groups to the surface, metal coating, and enclosure of foreign substances will lead to further expansion of single-walled nanotube applications.
Conventionally, the above-described single-walled nanotubes are produced by incorporating a metal such as iron, cobalt, nickel, and lanthanum into a cathode carbon rod and carrying out an arc discharge (Patent Document 1).
However, in this production method, in addition to single-walled nanotubes, the product also includes multi-walled nanotubes, graphite, and amorphous carbon. Consequently, not only is the yield low, but there is unevenness in the diameter and the length of the single-walled nanotubes. Thus, in this method, it is difficult to produce single-walled nanotubes having a comparatively even diameter and length at a high yield.
In addition to the above-described arc method, further examples of methods for producing carbon nanotubes include gas-phase pyrolysis, laser sublimation, and condensed phase electrolysis (Patent Documents 2 to 4).
However, the production methods described in these documents are all either carried out in the laboratory or at a small-scale level, and suffer from the problem that especially the carbon material yield is low.
Accordingly, the present applicant has previously proposed production apparatuses and methods of carbon nano fibers being nano-scale carbon materials which can be continuously mass produced using a fluidized bed reaction method (Patent Documents 5 to 8).